As a reading method of an image forming apparatus, there are two methods including fixed document reading and conveyed document reading. The fixed document reading is a method of loading a document on a contact glass, pressing the document against the contact glass with, for example, a platen mat or a document conveyance device, and reading the document by an image reading section. The conveyed document reading is a method of stopping the image reading section at a predetermined position and reading, with the image reading section, the document conveyed to this position.
In the conveyed document reading method, in a case where a contact glass for the fixed document reading and a contact glass for the conveyed document reading are provided separately, a typical configuration is provided such that a document discharge guide is put in below the contact glass for the conveyed document reading. Moreover, configuration such that one piece of contact glass is provided includes a configuration such that a transparent reading sheet is provided on a surface of the contact glass, image reading is performed through the transparent reading sheet, and part of the transparent reading sheet is loaded on a document discharge guide and such that document discharge is enabled or a configuration such that the transparent reading sheet is attached to the document reading device to permit document conveyance.
The transparent reading sheet is used to form a contact glass for the fixed document reading and the conveyed document reading with one piece for downsizing the device and reducing cost, and is used to read and discharge the document at time of the conveyed document reading. With such configuration, the document is read through the contact glass only in the fixed document reading while the document is read through the contact glass and the transparent reading sheet in the conveyed document reading, and thus image concentration varies depending on the reading method. In Patent Literature 1 described below, in order to match the image concentrations, in both the fixed document reading and the conveyed document reading, a reference document is read and a concentration difference is corrected.